[3.5] Adding item enhancements after the fact: is there a rule?

[OttoVonBigby]

Somewhere along the line I got the idea that an existing magic weapon/armor which already has a special ability cannot have a new special ability added-- i.e. that if I already have a +1 flaming longsword, I cannot pay 2000 gp to modify that same weapon into a +1 somethingbane flaming longsword--I would have to have a new weapon made (at a much greater cost).

The thing is, I can't find any such rule, and am beginning to suspect that there isn't one.

Anyone care to enlighten me? If 3.5 leaves this vague, how have you or your DMs handled it?

[Elderand]

There isn't such a rule, you can modify an existing weapon to your heart content as long as you don't get beyond the normal price tag.

In fact
From the SRD

Adding New Abilities
A creator can add new magical abilities to a magic item with no restrictions. The cost to do this is the same as if the item was not magical. Thus, a +1 longsword can be made into a +2 vorpal longsword, with the cost to create it being equal to that of a +2 vorpal sword minus the cost of a +1 sword.

[Matticussama]

You can add new abilities later on so long as you have the appropriate Craft Magic Item feats. You take the cost of the new enhancement, say +3 longsword, and subtract the value of the existing enhancement. So if you had a +1 longsword and wanted to improve it to +3, you take the total cost of the +3 minus the cost of the +1.

See the Adding New Abilities (http://www.d20srd.org/srd/magicItems/creatingMagicItems.htm) section.

Edit: Swordsage'd.

[Lord Il Palazzo]

Somewhere along the line I got the idea that an existing magic weapon/armor which already has a special ability cannot have a new special ability added-- i.e. that if I already have a +1 flaming longsword, I cannot pay 2000 gp to modify that same weapon into a +1 somethingbane flaming longsword--I would have to have a new weapon made (at a much greater cost).

The thing is, I can't find any such rule, and am beginning to suspect that there isn't one.

Anyone care to enlighten me? If 3.5 leaves this vague, how have you or your DMs handled it?The only thing close to that rule I'm aware of applies to adding the masterwork quality to a weapon, which might be what you were thinking about. Specifically (http://www.d20srd.org/srd/equipment/weapons.htm#masterworkWeapons):

You can’t add the masterwork quality to a weapon after it is created; it must be crafted as a masterwork weapon (see the Craft skill).
Note that going from a +1 flaming longsword to a +1 flaiming [something]bane longsword would cost more than 2000 GP. Flaming and bane are both +1 enhancements so it would cost 10000 GP going from a total of +2 in enhancements (at 8000 GP) to a total of +3 (at 18000 GP).